Robotic control of an endoscope from blood vessel tree images

ABSTRACT

A robot guiding system employs a robot unit ( 10 ) and a control unit ( 20 ). The robot unit ( 10 ) includes an endoscope ( 12 ) for generating an intra-operative endoscopic image ( 14 ) of a blood vessel tree within an anatomical region, and a robot ( 11 ) for moving the endoscope ( 12 ) within the anatomical region. The control unit ( 20 ) includes an endoscope controller ( 22 ) for generating an endoscopic path within the anatomical region, wherein the endoscopic path is derived from a matching of a graphical representation of the intra-operative endoscopic image ( 14 ) of the blood vessel tree to a graphical representation of a pre-operative three-dimensional image ( 44 ) of the blood vessel tree. The control unit ( 20 ) further includes a robot controller ( 21 ) for commanding the robot ( 11 ) to move the endoscope ( 12 ) within the anatomical region in accordance with the endoscopic path.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is a Continuation application of U.S. Ser. No.13/822,001, filed on Mar. 11, 2013, which is the U.S. National Phaseapplication under 35 U.S.C. §371 of International Application No.PCT/IB2011/053998, filed on Sep. 13, 2011 which claims the benefit ofU.S. Provisional Patent Application No. 61/382,980, filed Sep. 15, 2010.These applications are hereby incorporated by reference herein.

The present invention generally relates to robotic control of anendoscope during a minimally invasive surgical procedure (e.g., aminimally invasive coronary bypass grafting surgery). The presentinvention specifically relates to a matching of a graphicalrepresentation of a pre-operative three-dimensional (“3D”) blood vesseltree image to a graphical representation of an intra-operativeendoscopic blood vessel tree image as a basis for robotic guiding of anendoscope.

Coronary artery bypass grafting (“CABG”) is a surgical procedure forrevascularization of obstructed coronary arteries. Approximately 500,000operations are performed annually in the United States. In conventionalCABG, the patient's sternum is opened and the patient's heart is fullyexposed to a surgeon. Despite the exposure of the heart, some arteriesmay be partially invisible due to fatty tissue layer above them. Forsuch arteries, the surgeon may palpate the heart surface and feel bothblood pulsating from the arteries and a stenosis of the arteries.However, this data is sparse and might not be sufficient to transfer asurgical plan to the surgical site.

In minimally invasive CABG, the aforementioned problem of conventionalCABG is amplified because a surgeon cannot palpate the heart surface.Additionally, the length of surgical instruments used in minimallyinvasive CABG prevents any tactile feedback from the proximal end of thetool.

One known technique for addressing the problems with conventional CABGis to register an intra-operative site with a pre-operative 3D coronaryartery tree. Specifically, an optically tracked pointer is used todigitalize position of the arteries in an open heart setting and theposition data is registered to pre-operative tree using an IterativeClosest Point (“ICP”) algorithm known in art. However, this technique,as with any related approach matching digitized arteries andpre-operative data, is impractical for minimally invasive CABG becauseof spatial constraints imposed by a small port access. Also, thistechnique requires most of the arteries to be either visible or palpatedby the surgeon, which is impossible in minimally invasive CABG.

One known technique for addressing the problems with minimally invasiveCABG is to implement a registration method in which the heart surface isreconstructed using an optically tracked endoscope and matched topre-operative computer tomography (“CT”) data of the same surface.However, this technique, as with any related approach proposing surfacebased matching, may fail if the endoscope view used to derive thesurface is too small. Furthermore, as the heart surface is relativelysmooth without specific surface features, the algorithm of thistechnique more often than not operates in a suboptimal local maximum ofthe algorithm.

Another known technique for addressing the problems with minimallyinvasive CABG is to label a coronary tree extracted from a new patientusing a database of previously labeled cases and graph based matching.However, this technique works only if a complete tree is available andit's goal is to label the tree rather to match the geometry.

A further problem of minimally invasive CABG is an orientation and aguidance of the endoscope once the global positioning with respect topre-operative 3D images is reached. The goal of registration is tofacilitate localization of the anastomosis site and the stenosis. In astandard setup, the endoscope is being held by an assistant, while thesurgeon holds two instruments. The surgeon issues commands to theassistant and the assistant moves the endoscope accordingly. This kindof setup hinders hand-eye coordination of the surgeon, because theassistant needs to intuitively translate surgeon's commands, typicallyissued in the surgeon's frame of reference, to the assistant's frame ofreference and the endoscope's frame of reference. Plurality ofcoordinate systems may cause various handling errors, prolong thesurgery or cause misidentification of the coronary arteries.

A surgical endoscope assistant designed to allow a surgeon to directlycontrol an endoscope via a sensed movement of the surgeon head may solvesome of those problems by removing the assistant from the control loop,but the problem of transformation between the surgeon's frame ofreference and the endoscope's frame of reference remains.

The present invention provides methods for matching graphicalrepresentations of a blood vessel tree (e.g., furcation of arteries,capillaries or veins) as shown in a pre-operative three-dimensional(“3D”) image (e.g., a CT image, a cone beam CT image, a 3D X-Ray imagesor a MRI image) and in an intra-operative endoscopic image, overlayingthe blood vessel tree from the pre-operative 3D image to theintra-operative endoscopic image, and using the overlay to guide a robotholding an endoscope toward a location as defined in the pre-operative3D image.

One form of the present invention is a robotic guiding system employinga robot unit and a control unit.

A robot guiding system employs a robot unit and a control unit. Therobot unit includes an endoscope for generating an intra-operativeendoscopic image of a blood vessel tree within an anatomical region, anda robot for moving the endoscope within the anatomical region. Thecontrol unit includes an endoscope controller for generating anendoscopic path within the anatomical region, wherein the endoscopicpath is derived from a matching of a graphical representation of theintra-operative endoscopic image of the blood vessel tree to a graphicalrepresentation of a pre-operative three-dimensional image of the bloodvessel tree. The control unit further includes a robot controller forcommanding the robot to move the endoscope within the anatomical regionin accordance with the endoscopic path.

A second form of the present invention is a robot guiding methodinvolving a generation of an intra-operative endoscopic image of a bloodvessel tree within an anatomical region and a generation of anendoscopic path within the anatomical region, wherein the endoscopicpath is derived from a matching of a graphical representation of theintra-operative endoscopic image of the blood vessel tree to a graphicalrepresentation of a pre-operative three-dimensional image of the bloodvessel tree. The robot guiding method further involves a commanding of arobot to move an endoscope within the anatomical region in accordancewith the endoscopic path.

The term “pre-operative” as used herein is broadly defined to describeany activity executed before, during or after an endoscopic imaging ofan anatomical region for purposes of acquiring a three-dimensional imageof the anatomical region, and the term “intra-operative” as used hereinis broadly defined to describe any activity executed by the robot unitand the control unit during an endoscopic imaging of the anatomicalregion. Examples of an endoscopic imaging of an anatomical regioninclude, but are not limited to, a CABG, a bronchoscopy, a colonscopy, alaparascopy, and a brain endoscopy.

The foregoing forms and other forms of the present invention as well asvarious features and advantages of the present invention will becomefurther apparent from the following detailed description of variousembodiments of the present invention read in conjunction with theaccompanying drawings. The detailed description and drawings are merelyillustrative of the present invention rather than limiting, the scope ofthe present invention being defined by the appended claims andequivalents thereof.

FIG. 1 illustrates an exemplary embodiment of a robotic guiding systemin accordance with the present invention.

FIG. 2 illustrates a flowchart representative of an exemplary embodimentof a robotic guidance method in accordance with the present invention.

FIG. 3 illustrates an exemplary surgical implementation of the flowchartshown in FIG. 2

FIG. 4 illustrates a flowchart representative of an exemplary embodimentof a graph matching method in accordance with the present invention.

FIGS. 5 and 6 illustrate an exemplary ordering of main graphs of a bloodvessel tree in accordance with the present invention.

FIG. 7 illustrates an exemplary overlay of geometrical representation onan endoscopic image accordance with the present invention.

FIG. 8 illustrates an exemplary robot paths within the overlay shown inFIG. 7 in accordance with the present invention.

As shown in FIG. 1, a robotic guiding system employs a robot unit 10 anda control unit 20 for any endoscopic procedure involving an endoscopicimaging of a blood vessel tree having one or more furcations (i.e.,branches). Examples of such endoscopic procedures include, but are notlimited to, minimally invasive cardiac surgery (e.g., coronary arterybypass grafting or mitral valve replacement).

Robot unit 10 includes a robot 11, an endoscope 12 rigidly attached torobot 11 and a video capture device 13 attached to the endoscope 12.

Robot 11 is broadly defined herein as any robotic device structurallyconfigured with motorized control of one or more joints for maneuveringan end-effector as desired for the particular endoscopic procedure. Inpractice, robot 11 may have four (4) degrees-of-freedom, such as, forexample, a serial robot having joints serially connected with rigidsegments, a parallel robot having joints and rigid segments mounted inparallel order (e.g., a Stewart platform known in the art) or any hybridcombination of serial and parallel kinematics.

Endoscope 12 is broadly defined herein as any device structurallyconfigured with ability to image from inside a body. Examples ofendoscope 12 for purposes of the present invention include, but are notlimited to, any type of scope, flexible or rigid (e.g., endoscope,arthroscope, bronchoscope, choledochoscope, colonoscope, cystoscope,duodenoscope, gastroscope, hysteroscope, laparoscope, laryngoscope,neuroscope, otoscope, push enteroscope, rhinolaryngoscope,sigmoidoscope, sinuscope, thorascope, etc.) and any device similar to ascope that is equipped with an image system (e.g., a nested cannula withimaging). The imaging is local, and surface images may be obtainedoptically with fiber optics, lenses, and miniaturized (e.g. CCD based)imaging systems.

In practice, endoscope 12 is mounted to the end-effector of robot 11. Apose of the end-effector of robot 11 is a position and an orientation ofthe end-effector within a coordinate system of robot 11 actuators. Withendoscope 12 mounted to the end-effector of robot 11, any given pose ofthe field-of-view of endoscope 12 within an anatomical regioncorresponds to a distinct pose of the end-effector of robot 11 withinthe robotic coordinate system. Consequently, each individual endoscopicimage of a blood vessel tree generated by endoscope 12 may be linked toa corresponding pose of endoscope 12 within the anatomical region.

Video capture device 13 is broadly defined herein as any devicestructurally configured with a capability to convert an intra-operativeendoscopic video signal from endoscope 12 into a computer readabletemporal sequence of intra-operative endoscopic image (“IOEI”) 14. Inpractice, video capture device 13 may employ a frame grabber of any typefor capturing individual digital still frames from the intra-operativeendoscopic video signal.

Still referring to FIG. 1, control unit 20 includes a robot controller21 and an endoscope controller 22.

Robot controller 21 is broadly defined herein as any controllerstructurally configured to provide one or more robot actuator commands(“RAC”) 26 to robot 11 for controlling a pose of the end-effector ofrobot 11 as desired for the endoscopic procedure. More particularly,robot controller 21 converts endoscope position commands (“EPC”) 25 fromendoscope controller 22 into robot actuator commands 26. For example,endoscope position commands 25 may indicate an endoscopic path leadingto desired 3D position of a field-of-view of endoscope 12 within ananatomical region whereby robot controller 21 converts command 25 intocommands 26 including an actuation current for each motor of robot 11 asneeded to move endoscope 12 to the desired 3D position.

Endoscope controller 22 is broadly defined herein as any controllerstructurally configured for implementing a robotic guidance method inaccordance with the present invention and exemplary shown in FIG. 2. Tothis end, endoscope controller 22 may incorporate an image processingmodule (“IPM”) 23, which is broadly defined herein as any modulestructurally configured for executing an anatomical object imageregistration of the present invention. In particular, a blood vesseltree image registration as exemplarily implemented by stages S32 and S33of flowchart 30 shown in FIG. 2. Endoscope controller 22 may furtherincorporate a visual servo module (“VSM”) 24, which is broadly definedherein as any module structurally configured for generating endoscopeposition commands 25 indicating an endoscopic path leading to desired 3Dposition of a field-of-view of endoscope 12 within an anatomical region.In particular, endoscope position commands 25 are derived from the bloodvessel tree image registration as exemplarily implemented by a stage S34of flowchart 30 shown in FIG. 2.

A description of flowchart 30 will now be provided herein to facilitatea further understanding of endoscope controller 22.

Referring to FIG. 2, a stage S31 of flowchart 30 encompasses anextraction of a geometrical representation of a blood vessel tree from apre-operative 3D image. For example, as shown in FIG. 3, a 3D imagingdevice (e.g., a CT device, an X-ray device, or a MRI device) is operatedto generate a pre-operative 3D image 42 of a chest region of a patient50 illustrating left and right coronary arteries 51 and 52 of patient50. Thereafter, a blood vessel tree extractor 43 is operated to extracta geometrical representation 44 of a coronary arterial tree from image42, which may be stored in a database 45. In practice, a Brilliance iCTscanner sold by Philips may be used to generate image 42 and to extracta 3D dataset of the coronary arterial tree from image 42.

Referring back to FIG. 2, a stage S32 of flowchart 30 encompasses imageprocessing module 23 matching the graphical representation of one ormore intra-operative endoscopic images 14 (FIG. 1) of the blood vesseltree to a graphical representation of pre-operative 3D image 44 (FIG. 1)of the blood vessel tree. For example, as shown in FIG. 3, endoscope 12generates an intra-operative endoscopy video of a chest region ofpatient 50 that is captured by video capture device 13 and convertedinto intra-operative endoscopic images 14 whereby image processingmodule 23 of endoscope controller 22 matches a graphical representationof the intra-operative endoscopic image(s) 14 of the coronary arterialtree to a graphical representation of pre-operative 3D image 44 of thecoronary arterial tree. In one exemplary embodiment, image processingmodule 23 executes a blood vessel tree image matching method of thepresent invention as exemplarily represented by a flowchart 60 shown inFIG. 4, which will be described herein in the context of the bloodvessel tree being a coronary arterial tree.

Referring to FIG. 4, a stage S61 of flowchart 60 encompasses imageprocessing module 23 generating a coronary arterial tree main graph froma geometrical representation of the coronary arterial tree in accordancewith any representation method known in the art. For example, as shownin stage S61, a geometrical representation 70 of a coronary arterialtree is converted into a main graph 71 having nodes represented of eachfurcation (e.g., a bifurcation or trifurcation) of coronary arterialtree geometrical representation 70 and further having branch connectionsbetween nodes. Stage S61 may be performed pre-operatively (e.g., daysbefore the endoscopic surgery or any time prior to an introduction ofendoscope 12 within patient 50), or intra-operatively by means of aC-arm angiography or other suitable system.

A stage S62 of flowchart 60 encompasses image processing module 23generating a coronary arterial tree subgraph from a portion of acoronary arterial tree visible in an intra-operative endoscopic image 14in accordance with any graphical representation method known in the art.Specifically, endoscope 12 is introduced into patient 50 whereby imageprocessing module 23 performs a detection of a coronary arterialstructure within the intra-operative endoscopic image 14. In practice,some arterial structures may be visible while other arterial structuresmay be hidden by a layer of fatty tissue. As such, image processingmodule 23 may implement an automatic detection of visible coronaryarterial structure(s) by known image processing operations (e.g.,threshold detection by the distinct red color of the visible coronaryarterial structure(s)), or a surgeon may manually use an input device tooutline the visible coronary arterial structure(s) on the computerdisplay. Upon a detection of the arterial structure(s), image processingmodule 23 generates the coronary arterial tree graph in a similar mannerto the generation of the coronary arterial tree main graph. For example,as shown in stage S62, a geometrical representation 72 of coronaryarterial structure(s) is converted into a graph 73 having nodesrepresented of each furcation (e.g., a bifurcation or trifurcation) ofcoronary arterial tree geometrical representation 72 and further havingbranch connections between nodes. Since both trees are coming from thesame person, it is understood that the graph derived from endoscopyimages is a subgraph of the graph derived from 3D images.

A stage S63 of flowchart 60 encompasses image processing module 23matching the subgraph to the maingraph in accordance with any knowngraph matching methods (e.g., maximum common subgraph or McGregor commonsubgraph). For example, as shown in stage S63, the nodes of subgraph 73are matched to a subset of nodes of main graph 71. In practice, subgraph73 may only be partially detected within intra-operative endoscopicimage 14 or some nodes/connections of subgraph 73 may be missing fromintra-operative endoscopic image 14. To improve upon the matchingaccuracy of stage S62, an additional ordering of main graph 71 andsubgraph 73 may be implemented.

In one embodiment, a vertical node ordering of main graph 71 isimplemented based on a known orientation of patient 50 during the imagescanning of stage S61. Specifically, the main graph nodes may bedirectionally linked to preserve a top-bottom order as exemplarily shownin FIG. 5 via the solid arrows. For subgraph 73, the orientation ofpatient 50 relative to endoscope 12 may not be known. However, knowingthat branches of the coronary arterial tree reduce in diameter as theyexpand top-bottom, then varying arterial sizes of the arterial branchesin intra-operative endoscopic image 14 may indicate orientation.

In another embodiment, a horizontal node ordering of main graph 70 maybe implemented based on the known orientation of patient 50 during theimage scanning of stage S61. Specifically, the main graph nodes may bedirectionally linked to preserve a left-right node order as exemplarilyshown in FIG. 6 via the dashed arrows. For subgraph 73, with theorientation of patient 50 to endoscope 12 more than likely beingunknown, the horizontal node order of subgraph 73 may be set by theoperating surgeon or an assistant via a graphical user interface.

While the use of ordering may decrease the time for matching the graphsand reduce the number of possible matches, theoretically multiplematches between the graphs may still be obtained by the matchingalgorithm. Such a case of multiple matches is addressed during a stageS33 of flowchart 30.

Referring again to FIG. 2, based on the matching of the graphs, a stageS33 of flowchart encompasses an overlay of the geometricalrepresentation of pre-operative 3D image 44 (FIG. 1) of the blood vesseltree on the intra-operative endoscopic image 14 of the blood vesseltree. This is done by using the geometrical representation uniquelyassociated to the maingraph. Thus, the entire geometry may be directlytranslated to intra-operative endoscopic image 14 using a perspectivetransformation. The perspective transformation may be detected fromintra-operative endoscopic image 14 and nodes in pre-operative 3D image44 using matching algorithms known in art, such as, of example,homography matching.

For example, FIG. 7 illustrates a geometrical representation 80 of acoronary arterial tree having nodes matched to nodes 91-95 with anintra-operative endoscopic image 90. The distance between each node pairamong nodes 91-95 may be used to determine a scaling factor forgeometrical representation 80 to thereby enable geometricalrepresentation 80 to overlay intra-operative endoscopic image 90 asshown.

In practice, if the graph matching of stage S32 (FIG. 2) yields multipleresults, then all possible overlays may be displayed to the surgeonwhereby the surgeon may select the matching result the surgeon believesis the most likely match via a graphical user interface. Given that thesurgeon knows the position of endoscope 12 relative to at least somestructures in intra-operative endoscopic image 14, the selection may berelatively straightforward.

Referring back to FIG. 2, a stage S34 of flowchart 30 encompasses visualservo module 32 generates an endoscopic path within the overlay of thegeometrical representation of pre-operative 3D image 44 (FIG. 1) of theblood vessel tree on intra-operative endoscopic image 14 (FIG. 1) of theblood vessel tree. Based on the endoscopic path, visual servo module 32generates endoscope position commands 25 to robot controller 21 tothereby guide endoscope 12 (FIG. 1) along the endoscopic path to adesired position within the anatomical region. Specifically, once theexact overlay is found, robot 11 may be commanded to guide endoscope 12to positions the surgeon selects on pre-operative 3D image 44. Thesurgeon or the assistant may select a point of blood vessel tree, androbot 11 may guide endoscope 12 towards that desired position along anysuitable path. For example, as shown in FIG. 9, robot 11 may moveendoscope 12 along a shortest path 101 to a desired position 100 oralong an coronary arterial path 102 to desired position 100. Coronaryarterial path 102 is the preferred embodiment, because coronary arterialpath 102 allows the surgeon to observe visible arteries as robot 11moves endoscope 12. In addition, it might help the surgeon to decide ifthe matching was successful. Coronary arterial path 102 may be definedusing methods known in art (e.g., Dijkstra shortest path algorithm).

In practice, the movement of robot 11 may be commanded usinguncalibrated visual servoing with remote center of motion, and the fieldof view of endoscope 12 may be extended to enable a larger subgraphduring matching stage S32.

Referring back to FIG. 2, stages S32-S34 may either be executed onetime, or on a periodical basis until such time robot 11 has movedendoscope 12 to the desired position within the anatomical region, ormultiple times as dictated by the surgeon.

In practice, modules 23 and 24 (FIG. 1) may be implemented by hardware,software and/or firmware integrated within endoscope controller 22 asshown.

From the description of FIGS. 1-8 herein, those having ordinary skill inthe art will appreciate the numerous benefits of the present inventionincluding, but not limited to, an application of the present inventionto any type of endoscopy surgery performed on any type of blood vessels.

Although the present invention has been described with reference toexemplary aspects, features and implementations, the disclosed systemsand methods are not limited to such exemplary aspects, features and/orimplementations. Rather, as will be readily apparent to persons skilledin the art from the description provided herein, the disclosed systemsand methods are susceptible to modifications, alterations andenhancements without departing from the spirit or scope of the presentinvention. Accordingly, the present invention expressly encompasses suchmodification, alterations and enhancements within the scope hereof.

1. A control unit for controlling a surgical robot guiding systemincluding an endoscope and a robot configured to move the endoscopewithin an anatomical region exterior to blood vessels of a blood vesseltree, the control unit comprising: an endoscope controller configured togenerate an endoscopic path exterior to the blood vessels within theanatomical region, wherein the endoscopic path is derived from matchinga graphical representation of intra-operative endoscopic image of thesubregion of the blood vessel tree to a graphical representation of apre-operative three-dimensional image of the blood vessel tree; and arobot controller configured to command the robot to move the endoscopewithin the anatomical region exterior to the blood vessels in accordancewith the endoscopic path.
 2. The control unit of claim 1, wherein thematching of the graphical representation of the intra-operativeendoscopic image of the blood vessel tree to the graphicalrepresentation of the pre-operative three-dimensional image of the bloodvessel tree includes: generating a main graph derived from thegeometrical representation of the pre-operative three-dimensional imageof the blood vessel tree; generating a subgraph derived from ageometrical representation of the intra-operative endoscopic image ofthe subregion of the blood vessel tree; and matching the subgraph to themain graph.
 3. A control unit for controlling a surgical robot guidingsystem including an endoscope and a robot configured to move theendoscope within an anatomical region exterior to blood vessels of ablood vessel tree, the control unit comprising: an endoscope controllerconfigured to generate an endoscopic path exterior to the blood vesselswithin the anatomical region, wherein the endoscopic path is derivedfrom matching a graphical representation of the intra-operativeendoscopic image of the subregion of the blood vessel tree to agraphical representation of a pre-operative three-dimensional image ofthe blood vessel tree, wherein the matching of the graphicalrepresentation of the intra-operative endoscopic image of the bloodvessel tree to the graphical representation of the pre-operativethree-dimensional image of the blood vessel tree includes: generating amain graph derived from the geometrical representation of thepre-operative three-dimensional image of the blood vessel tree, whereinthe main graph includes a main set of nodes representative of eachfurcation of the blood vessel tree within the pre-operativethree-dimensional image of the blood vessel tree, generating a subgraphderived from a geometrical representation of the intra-operativeendoscopic image of the subregion of the blood vessel tree, wherein thesubgraph includes a subset of the main set of nodes, the subset of nodesbeing representative of each furcation of the blood vessel tree withinthe intra-operative endoscopic image of the blood vessel tree, andmatching the subgraph to the main graph; a robot controller configuredto command the robot to move the endoscope within the anatomical regionexterior to the blood vessels in accordance with the endoscopic path. 4.The control unit of claim 3, wherein the matching of the subgraph to themain graph includes: establishing at least one of a vertical orderingand a horizontal ordering of the nodes in the main graph.
 5. The controlunit of claim 2, wherein the endoscope controller is further configuredto overlay the geometrical representation of the pre-operativethree-dimensional image of the exterior of the blood vessel tree ontothe intra-operative endoscopic image of the subregion of the bloodvessel tree corresponding to the subgraph in accordance with thematching of the graphical representation of the intra-operativeendoscopic image of the blood vessel tree to the graphicalrepresentation of the pre-operative three-dimensional image of the bloodvessel tree.
 6. The control unit of claim 5, wherein the endoscopecontroller is further configured to generate the endoscopic path withinthe geometrical representation of the pre-operative three-dimensionalimage of the exterior of the blood vessel tree overlaid onto theintra-operative endoscopic image of the corresponding portion of theblood vessel tree.
 7. The control unit of claim 2, wherein the matchingof the subgraph to the main graph includes matching nodes of thesubgraph to nodes of subregions of the main graph to locate one or morecandidate subregions of the main graph corresponding to the subgraph;and wherein one of the plurality of the candidate subregions locatedfrom the matching is selected as a match of the subgraph to the maingraph.
 8. The control unit of claim 1, wherein the blood vessel tree isa coronary artery tree.